The present invention relates to a method of producing powder-metallurgical objects, specifically elongate objects such as rods, sections, tubes or the like, wherein a powder of metal/and or metal alloys of great hardness, specifically tool or high-speed steel powder, is charged into a thin-walled capsule, said capsule is then sealed so as to be airtight, is heated and subjected to isostatic pressure so as to produce a blank which will then be hot-worked, particularly extruded, to form the finished product.
Such a method is known, for instance, from the German Patent DE-C-35 30 741. Tests have shown that the known method cannot be employed for the use of tool steel or high-speed steel powder to produce a flawless, specifically crack-free finished product. The reason is to be found in the low plasticity and malleability of the metal powder used which provides for a maximum powder density of 75% of the theoretical value under the common conditions of cold isostatic compression. Due to the hardness of the metal powder here used, the isostatic compression does not allow for a tight bonding between the material of the capsule and the adjoining metal powder, which results in the formation of folds or "wrinkles" in the comparatively soft material of the capsule in the extrusion step. This phenomenon leads to surface cracks and fissures in the finished product which will become apparent after removal of the capsule material.
Knowing the problems described above, experts have proposed, in the Swedish Patent SE-A-442 486, to heat a high-speed steel powder initially to a temperature between 850.degree. C. and 900.degree. C., in a non-oxidizing environment. Then the thus obtained "powder cake" is pulverized again by mechanical means, i.e. by grinding. Then the powder is reheated to a temperature of 850.degree. C. to 900.degree. C. A shaping tool is then used to press the thus pretreated powder into a blank which will be sintered subsequently at a temperature between 1,150.degree. C. and 1,250.degree. C. until the so-called "communicating porosity" in the blank will have been eliminated, which means that a closed-pore blank is produced. This step is followed by a warm isostatic final compression to a density of practically 100% of the theoretical value. It is obvious that this known method of producing powder-metallurgical objects involves a comparatively expensive process which is not suitable anyhow for the production of elongate objects.
The Australian Patent AT-A-377 718 describes a method of manufacturing objects made of tool steel powder, wherein the powder is charged into a capsule, this capsule is sealed so as to be airtight, is then heated, and wherein the airtight and sealed capsule is heated to a temperature between 700.degree. and 1,000.degree. C., the introduction of air into the capsule is permitted again, and the capsule is then heated to a temperature between 1,050.degree. C. and 1,200.degree. C. This known method obviously still requires that the extruded rods be annealed for several hours whereupon they are continuously cooled, initially in steps to a lower temperature and subsequently in ambient air without temperature control. This method, too, is rather expensive and troublesome, while it appears to be problematic to open the capsule again after heating the same to a temperature between 700.degree. C. and 1,000.degree. C. for heating it to an even higher temperature. This method involves an increased risk of oxidation so that the suitability of this method for practical realization well appears doubtful.